1. Field of the Invention (Technical Field)
The present invention relates to carbon-coated nanoparticles and carbon nanospheres and methods of manufacture of same.
2. Description of Related Art
Metallic nanoparticles exhibit a unique set of properties, considerably different than the bulk structures due to their large surface areas. They have a variety of applications, including energetic materials, optical and magnetic materials, electronic devices, and solid state chemical sensors. Existing methods of producing these particles include electric arc discharge, metal gas evaporation, metal evaporation in a flowing gas stream, mechanical attrition, sputtering, electron beam evaporation, electron beam induced atomization of binary metal azides, expansion of metal vapor in a supersonic free jet, inverse micelle techniques, laser ablation, laser-induced breakdown of organometallic compounds, pyrolysis of organometallic compounds, and microwave plasma decomposition of organometallic compounds.
A factor that limits use of metallic nanoparticles is their air-sensitivity. Metals form a passivation layer upon exposure to oxygen and, in some cases, become pyrophoric. The passivation layers that form on nanoparticles are often comparable in size to the size of the particle itself. For example, Applicants have determined that aluminum nanoparticles ranging in diameter from approximately 2 to 50 nm form a passivation layer approximately 4 nm thick. On a 25 nm particle, the passivation layer thus represents nearly 70% of the volume of the particle.
It is therefore desirable to produce nanoparticles that maintain their metallic properties while also minimizing or eliminating the passivation layer. One method to achieve this goal is by coating the particle with a thin layer of carbon, preferably in the form of graphite. Nanoparticles formed via electric arc discharge between metal-filled graphite electrodes have been encapsulated in a graphite 10-80 nm thick. These particles were resistant to oxidation. R. Seshadri, et al., Chem. Phys. Lett. 231:308-313 (1994); see also P. Nolan, et al., “Graphite encapsulation of catalytic metal nanoparticles”, Carbon 34:817 (1996).
The present invention is of an improved manufacturing method wherein one can form carbon-coated nanoparticles in an atmospheric pressure microwave plasma torch, a much less costly, more robust, and more productive solution. The invention is also of hollow carbon nanospheres which are formed by first making carbon-coated metal nanoparticles and then removing the metal.